A photoelectric conversion device having, as an organic photoelectric conversion layer, a mixture of an organic photoelectric conversion dye and a fullerene, described, for example, in JP-A-2001-7366 is attracting attention. A solar cell that receives sunlight and produces an electric energy is remarkably spreading as an apparatus for producing a clean energy replacing the petroleum energy, and above all, a solar cell having, as an organic photoelectric conversion layer, a mixture of an organic photoelectric conversion dye and a fullerene is highlighted thanks to its advantage that the photoelectric conversion efficiency is high.
However, such an organic photoelectric conversion layer has a problem that in the initial stage of receiving light irradiation, the photoelectric conversion efficiency is low.
In an imaging device for taking a subject image, individual pixels have a photoelectric conversion device, but the conventional imaging device called a CCD-type image sensor or a CMOS-type image sensor is approaching the limit of its production due to microfabrication and facing various problems. These problems are described below.
An image sensor (imaging device) in the related art mounted, for example, in a digital still camera, a digital video camera, a camera for cellular phones and a camera for endoscopes is configured such that pixels containing a photodiode as a photoelectric conversion device are formed and arranged in a two-dimensional array manner on a semiconductor substrate such as silicon chip and a signal charge corresponding to a photoelectron generated in the photodiode of each pixel is read to the outside through a CCD-type or CMOS-type signal readout circuit formed in the same semiconductor substrate.
As just described, in the image sensor in the related art, not only a photodiode as a photoelectric conversion device but also, for example, a signal readout circuit and a multilayer wiring associated therewith are formed together on a semiconductor substrate, and as the miniaturization of a pixel proceeds, the region occupied by the signal readout circuit or wiring in one pixel becomes relatively large, raising a problem of “reduction of aperture ratio”, that is, the light-receiving area of the photodiode on one chip being relatively decreased.
The reduction of aperture ratio leads to reduction in sensitivity, and it becomes difficult to take a bright image or a low-noise image. To solve this problem, “a photoelectric conversion layer-stacked solid-state imaging device” where a signal readout circuit, wiring and the like are formed in a semiconductor substrate and a photoelectric conversion layer is stacked above the semiconductor substrate, thereby increasing the aperture ratio, has been proposed, for example, in JP-B-1-34509.
The photoelectric conversion layer-stacked solid-state imaging device is fabricated, for example, by forming and arranging a plurality of pixel electrode films in a two-dimensional array manner on a semiconductor substrate having formed therein a signal readout circuit and wiring, stacking a photoelectric conversion layer thereon as a monolithic configuration, and further forming a transparent opposite electrode film thereon as a monolithic configuration. In some cases, the opposite electrode film is disposed on the semiconductor substrate side and the pixel electrode film is disposed on the light entering side. In this case, the pixel electrode film is a transparent electrode film.
In such a photoelectric conversion layer-stacked solid-state imaging device, when a bias voltage is applied between the pixel electrode film and the opposite electrode film, an exciton generated in the photoelectric conversion layer by receiving incident light is dissociated into an electron and a hole, and a signal corresponding to an electron or hole charge migrated to the pixel electrode film in accordance with the bias voltage is output as a taken-in image signal to the outside of the imaging device through a CCD-type or CMOS-type signal readout circuit provided in the semiconductor substrate.
A conventional technique using an organic semiconductor for the photoelectric conversion layer is introduced, for example, in JP-A-2008-72090 and JP-A-2007-273945. The photoelectric conversion layer including an organic semiconductor has a large light absorption coefficient and enables formation as a thin film and thanks to little diffusion of an electric charge to the adjacent pixel, optical color mixing and electrical color mixing (cross-talk) can be reduced.
However, the photoelectric conversion device containing a P-type organic semiconductor and an N-type organic semiconductor described in JP-B-1-34509, JP-A-2008-72090, and JP-A-2007-273945 has a problem in view of durability, for example, the structure readily deteriorates, and there is room for improvement in the deterioration of sensitivity upon light irradiation.
On the other hand, in the conventional technique described in Japanese Patent No. 3,986,697, a structure formed by stacking a fullerene deposited film and a fullerene polymer film is used for the photoelectric conversion layer provided between a pixel electrode film and an opposite electrode film, and deterioration of the film structure due to unstable bonding attributable to intermolecular force of C60 molecules of the fullerene deposited film is overcome by the fullerene polymer film.
According to this conventional technique, durability of the fullerene thin film is improved while utilizing its characteristics, and a photoelectric conversion device holding a room temperature operable performance and at the same time, being physically and chemically stable can be obtained.
However, in the conventional technique of Japanese Patent No. 3,986,697, the fullerene polymer film is used only for suppressing the deterioration of film structure of the fullerene deposited film, and there is a problem that a high photoelectric conversion efficiency cannot be obtained at the initial stage of operation of the photoelectric conversion device.
Furthermore, in the conventional technique of JP-A-2004-165609, the pn junction is created by using a fullerene molecule monomer-containing layer for the n-type semiconductor layer and a fullerene molecule polymer-containing layer for the p-type semiconductor layer, but this also has a problem that a high photoelectric conversion efficiency cannot be obtained at the initial stage of operation of the photoelectric conversion device.